Rapid fuse casting refractory shapes in liquid cooled metal molds

ABSTRACT

A CHARGE OF MOLTEN REFRACTORY OXIDES, SUCH AS MIXTURES OF MAGNESIA AND CHROMIUM OXIDE, CONTAINING A GAS LIBERATING INGREDIENT ARE POURED INTO A LIQUID COOLED MOLD VIA A SWIRLING CASTING JET IN A FUNNEL WHERE SMALL FRAGMENTS OF FUSED CAST SCRAP OF LIKE COMPOSITION ARE ADDED   AND HOMOGENEOUSLY MIXED WITH THE MELT. THE SOLID FRAGMENTS, THE GAS LIBERATING INGREDIENT AND THE LIQUID COOLED METAL MOLD COOPERATE TO CAUSE A SOLIDIFIED SHELL TO FORM NEXT TO THE MOLD SURFACE OF SUCH A THICKNESS THAT THE PART CAN BE REMOVED FROM THE MOLD WITHIN A FEW MINUTES AFTER CASTING AND THEREAFTER COOLED OUTSIDE THE MOLD AT A CONVENTIONAL RATE WITHOUT CRACKING OR DISTORTION OF SHAPE.

Oct. 2, 1973 DUCHENOY ETAL 3,753,302

RAPID FUSE CASTING REFRACTORY SHAPES IN LIQUID 000mm METAL MOLDS 2 Sheets-Sheet 1 Filed Oct. 9, 1970 Oct. 2, 1973 J. DUCHENOY r-rrAL 3,753,302 RAPID FUSE CASTING REFRACTORY SHAPES IN LIQUID COOLED METAL MOLDS Filed Oct. 9, 1970 2 Sheets-Sheet 2 cracks d Well t/mes I 2 in the mould FIG. :6

defurm ions 1 cracks dwell times 7 7mn in the mould l 3mn United States Patent Oflice 3,763,302 Patented Oct. 2, 1973 3,763,302 RAPID FUSE CASTING REFRACTORY SHAPES IN LIQUID COOLED METAL MOLDS Jacques Duchenoy, Guy Gasparini, and Yvon Marty, Vedene, France, assignors to LElectro-Rcfractaire, Paris, France Filed Oct. 9, 1970, Ser. No. 79,439 Claims priority, application France, Oct. 10, 1969, 6934715; May 28, 1970, 7019538 Int. Cl. C04b 35/04, 35/12, 35/62 US. (:1. 264332 3 Claims ABSTRACT OF THE DISCLOSURE A charge of molten refractory oxides, such as mixtures of magnesia and chromium oxide, containing a gas liberating ingredient are poured into a liquid cooled mold via a swirling casting jet in a funnel where small fragments of fused cast scrap of like composition are added and homogeneously mixed with the melt. The solid fragments, the gas liberating ingredient and the liquid cooled metal mold cooperate to cause a solidified shell to form next to the mold surface of such a thickness that the part can be removed from the mold within a few minutes after casting and thereafter cooled outside the mold at a conventional rate without cracking or distortion of shape.

This invention relates to the manufacture of molded parts from molten refractory oxides.

The manufacture of moldings from previously molten refractory oxides, such as alumina, zirconia, chromium oxide, magnesia, etc., all materials with a very high melting point, have always presented technical problems.

One of these problems is the constitution of the walls of the mold, as these walls must tolerate the very high temperature of the material when it is poured, without being destroyed.

The material most generally used is graphite, and the mold then consists of graphite plates assembled among themselves. However such molds are not entirely satisfactory because upon contact with the poured material the graphite burns to some extent, so that after a few molding operations the molds become unserviceable and have to be replaced, which naturally involves an increase in the cost price.

Furthermore, if as is frequently practised, the material is left in the mold throughout its solidification and cooling, which must be fairly slow to prevent cracking, it is a long time before the molds can be re-used and the manufacturer is compelled to have a large number of molds available.

It has also been proposed recently to use opening molds with metal walls suitably cooled to withstand the temperature of the poured oxide material without melting.

Molds of this type can be more durable than graphite molds, and the ingot can be removed after adequate superficial solidification, to undergo controlled cooling and complete solidification elsewhere. These molds therefore appear to solve the re-use problem. Nevertheless they raise other difficulties owing to the appearance of cracks on the molded ingot, these cracks being due to excessively rapid cooling of the superficial part in contact with the cooled metal walls.

If attempts are made to overcome this disadvantage by removing the ingot from the mold after a very short time, another disadvantage is encountered, because the ingot, which consists of a solidified skin or crust in contact with the mold and a still liquid centre, with very high temperature gradients, deforms when it is no longer supported by the walls of a mold.

This disadvantage occurs more particularly when the molding of parts with relatively large dimensions is attempted.

On the other hand another problem which is encountered in the molding of refractory oxide poured in the molten state, originates from the shrinkage hole or cavity which is formed in the part during solidification, beneath the pouring funnel, and which is due mainly to the shrinkage of the material during solidification. This shrinkage hole may have a considerable volume.

It is a traditional practice to allow the liquid level to rise into the pouring funnel at the end of pouring, so as to supply the available volume in the mold to the fullest degree.

With the same object, and in a more rational manner, devices have also been proposed for the purpose of supplying and filling the shrinkage hole progressively as it is formed, by introducing a complementary charge maintained molten in the pouring funnel, whereas the material occupying the top of the mold is likewise maintained liquid. However such devices are complicated.

For this reason, a frequent practice is to mold a larger volume than the volume required for the parts to be manufactured, then after solidification and removal from the mold, eliminating by sawing or fracture the top part of the molding which contains the shrinkage hole. However this procedure involves an additional operation, whilst it also involves substantial waste.

In order to reduce wastage to a minimum, a mold is frequently used which has a large volume, which is a multiple of the volume of the part to be manufactured plus the volume of the part containing the shrinkage hole. By successive sawing operations, the said part is eliminated and the sound part is cut up in order to obtain the required elementary parts.

Such a procedure is still complicated and expensive. Furthermore, the cutting produces parts which on the one hand may differ from one another according to their position in the ingot, and on the other hand have two sawn faces which may be more sensitive to physico-chemical changes during their use as linings for furnaces and other appliances, than those faces in contact With the mold which are covered with their solidification skin.

The problems which have just been recapitulated become even more serious when dealing with magnesia or with mixtures containing a high proportion of this oxide, and frequently chromium oxide, and which are generally intended to form the linings of metallurgical appliances. Magnesia has an extremely high melting point of the order of 2,800" C., and produces a large shrinkage cavity in the customary processes. Since this shrinkage cannot be tolerated in the relevant application, the above-mentioned methods were used chiefly with this oxide.

Furthermore, the high temperature of this oxide would appear to be unfavourable to the use of molds made of steel or copper, or at least to dictate a very rapid removal of the ingot with increased risk of deformation.

The applicant has however succeeded in developing a method of manufacturing molded parts, unit by unit, from molten refractory oxides and more particularly from mixtures of magnesia and chromium oxide, in a cooled metallic mold, preferably a copper mold.

This method, which was not obvious, and is the culmination of thorough research, constitutes one of the subjects of the invention.

It enables a number of valuable results to be obtained, under conditions of rapid re-use of the mold, thanks to the rapid removal of the ingot, which is therefore freer from cracks, whilst enabling parts to be obtained of good shape, with no concentrated shrinkage hole and uniformly covered with a strong solidification crust, the regularity of the characteristics of the parts, the elimination of difiicult machining operations and the reduction of waste to a minimum, resulting in economy.

Furthermore, the invention makes it possible to obtain, for the fabrication of linings for metallurgical appliances based on magnesia and chromium oxides, homogeneous parts without sawn joint faces, giving the linings made from them good resistance to the destructive action of physico-chemical agents.

According to one special feature of the invention, small calibrated solid fragments of refractory oxide from the scrap of previous molding operations are added to the pouring jet progressively with its introduction into the mold.

The applicants have found experimentally that it is possible in this way to avoid deformations of ingot despite very rapid removal from the mold dictated by the necessity to avoid superficial cracking.

This surprising result is no doubt due to the fact that the solid fragments in sufficient quantity, distributed in the centre of the poured material, create therein an equal number of distributed cooling centres which contribute to the absence of deformation despite rapid removal from the mold.

Moreover, the said solid fragments distributed in the poured material contribute to the fragmentation of the solidification shrinkage and to the dispersion of the said shrinkage in the volume of the ingot, which is also an advantage.

Another special feature of the invention consists in that the charge to be melted contains at least one substance capable of liberating gases, so that the gas bubbles which are formed are distributed in the poured material and also contribute to combat the formation of a concentrated shrinkage cavity. I

Preferably, contrary to the customary practices and tendencies, the mold is filled only up to a specific level, ensuring that the level of the liquid does not rise, or rises only very slightly, into the bottom of the funnel or other pouring utensil, which is removed shortly after the pouring jet has been stopped.

From the commencement of external solidification of the part, and after a time of the order of a few minutes, the mold is opened and the part is removed to undergo its controlled cooling outside the mold.

The parts thus produced, being molded to their final dimensions, do not require machining, except possibly to eliminate from the pouring face the irregularities which may remain after the funnel is removed.

The two above stated measures: addition of solid fragments during the pouring and presence in the charge to be melted of a substance capable of generating gases, together produce very favourable effects for the particular end in view.

Their simultaneous application also provides a possibility of regulating the process, as will be explained at the end of the detailed description hereinbelow.

The description hereinbelow, given with reference to the accompanying drawing and by way of non-limitative example, will clearly show how the invention may be performed, and the special features which emerge both from the drawing and from the text are of course part of the said invention.

FIG. 1 is a schematic view in elevation and in vertical section of a mode of construction of an opening mold used according to the invention;

FIG. 2 shows the same mold in cross section along the line IIII;

FIG. 3 shows the pouring funnel in horizontal projection and illustrates the pouring operation;

FIG. 3 shows the pouring funnel in horizontal projection and illustrates the pouring operation;

FIG. 4 shows the part removed from the mold to be carried to the annealing or controlled cooling oven;

FIGS. 5 and 6 Show two explanatory graphs;

FIG. 7 shows in perspective a finished molding, the part having been sawn through a plane so as to show its internal structure.

According to the invention, it being proposed to manufacture, e.g. refractory bricks or arch stones to constitute the linings of metallurgical appliances, using a mixture of refractory oxides containing a high proportion of magnesia and chromium oxide Cr O which constitutes the most important application of the invention, the following or a similar procedure is adopted.

A charge suitable for obtaining a final product having the following composition is melted in the electric furnace at a temperature of the order 2,500 0:

Percent MgO Approximately 53-56. Cr O Approximately 23. FeO Approximately 10-12. SiO Approximately 1.5-2.7. CaO Approximately 1.2-1.5. A1 0 Approximately 7-10.

This charge contains substances capable of giving off gases.

For example, for the chromium oxide generally introduced into the charge in the state of chromite (chrome ore) it is possible to use, besides chromite from the Transvaal, a certain proportione.g. one-half-of Iranian or Turkish chromite, which gives off gases at the pouring temperature (this chromite undergoes an ignition loss of over 1%).

It is also possible to add the lime in the state of sulphate or carbonate, or partly magnesia in the state of sulphate or carbonate, or again hydrated magnesia, such as brucite, or to combine several of these additions among themselves.

Since the aim in view is the presence in the poured material of distributed gas bubbles capable of inhibiting the formation of a concentrated shrinkage hole in the solidified part, the quantity of gas generating material can be found experimentally in each case.

Part of the gases is given off in the melting furnace and is lost, but another part remains liquid, possibly in the dissolved state, and forms bubbles during cooling in the mold, which is accelerated by the addition of calibrated solid fragments as will be explained hereinbelow.

After having been molten in the electric furnace, the charge (further reference to its composition will be made hereinbelow) is poured into a mold, one form of construction of which given by way of example is illustrated in FIGS. 1 and 2. This mold, the volume of which is parallelepipedic or prismatic, and is thus suitable for manufacturing bricks or arch stones for lining walls, comprises a movable bottom 1, surmounted by a refractory plate 2 made of graphite, and four lateral walls 4, 5, 6, 7. These walls, relatively thick, are made of electrolytic copper. They are pierced by channels 8, in which is circulated a cooling liquid such as water delivered by a suitable pump.

The said walls, which at the moment of pouring are joined above the bottom 1, 2, are arranged so as to be capable of opening. They may for example, as the drawing shows be assembled into two opposite dihedrals integral with support arms 9, 10 adapted for pivoting about a vertical axis 11, the pivoting of the arms for closing the mold (direction of the arrows F) and also the pivoting in the opposite direction to open the mold (position shown by dash lines in FIG. 2) being driven by a hydraulic jack (not shown).

The bottom 1, 2 is vertically movable-cg. by means of a jack 2a, and can thus be moved outside the vertical axis of the mold.

The mold being in the assembled position, and two dihedrals being joined above the bottom 1, 2, the molten charge is poured into the mold through the intermediary of a pouring funnel 12 made of copper cooled by water (the cooling channels made in the wall of the funnel have not been shown), the lower part of which forms a plane flange 14 adapted to rest upon the top of the mold and to close the latter.

As FIG. 3 shows, the funnel is so arranged with reference to the nose 13 of the electric furnace, that the jet executes a rotation on the funnel before reaching the hole 15 of the latter and thus assumes a turbulent move ment.

Calibrated solid fragments 16 containing magnesia, preferably of the same composition as the jet, are dropped in the axis of the funnel. These fragments are entrained in the rotation of the jet and are distributed in the latter and in the mold.

This introduction is commenced at the outset of the pouring and it is continued regularly until the end of the pouring.

By way of example, satisfactory results have been obtained with calibrated fragments between and 20 mm. in size, of the same composition as the poured material, and falling into the pouring jet, throughout the pouring operation, at a constant rate of 9 kg. per minute for a liquid pouring rate of 60 to 70 kg. per minute.

The pouring and the introduction of the calibrated fragments are continued until the mold is full.

No attempt is made to fill the funnel, as is generally done in order to supply the shrinkage hole, and on the contrary the pouring is stopped as soon as the level of the liquid has risen slightly into the bottom of the funnel. The dropping of the calibrated fragments is stopped at the same time.

When the product has hardened in the narrow part of the funnel, the funnel is raised and removed and is then inverted so that the waste material falls out of it.

The mold is then opened by pivoting the arms 9 and 10 (in the opposite direction to the arrows F) so as to expose the ingot resting upon the pouring bottom 1, 2.

The mold bottom 1, 2 carrying the ingot is then lowered by means of its jack ('FIG. 4) and the ingot is taken by a tongs to be placed in an oven where it undergoes its controlled cooling for a time which may be of the order of 24 hours. But as soon as the part is removed, the mold is ready to be re-used for a fresh molding operation.

By way of example, for a part having the dimensions 150 x 150 x 600 mm, which corresponds to the industrial parts to be used as lining bricks, the pouring time is approximately 30 seconds, the funnel is lifted at the end of 2 minutes 45 seconds from the commencement of pouring, the mold being finally opened and the ingot removed 3 minutes after the commencement of pouring. The mold can be re-used immediately and its re-use time is therefore very short.

It is remarkable that parts removed so rapidly from the mold, when they are only covered by a solidified crust whereas their central part is still liquid and at a temperature of over 2,000 C., retain their shape.

As has been stated, this surprising result, found experimentally, can no doubt be explained by the fact that the calibrated solid fragments of refractory oxide which are present distributed throughout the volume of the ingot and which undergo only superficial melting, form, by virtue of the internal cooling centres which they constitute, kinds of bridges which hold the solidified crust whilst preventing it from being deformed by the internal thrusts of the liquid.

The said calibrated fragments also contribute to the rapid homogenisation of the temperatures within the block and also to the fragmentation of the solidification shrinkage, whilst the gas bubbles distributed in the mass of the ingot also play a part in this fragmentation.

The graph shown in FIGS. 5 and 6 express the experimental observations made by the applicant.

In these graphs, the dwell times of the molten material in the mold are plotted along the abscissa.

FIG. 5 illustrates what happens when the pouring is 6 performed without introducing the calibrated solid fragments into the jet.

The cracking of the moldings is observed when the dwell times in the mold are longer than t But on the other hand, the deformations of the ingot appear when they are removed from the mold after too short a time, and the absence of deformation would necessitate a dwell time in the mold longer than t Since t is longer than t it is thus impossible to avoid deformation and cracking simultaneously.

If, on the contrary, the calibrated solid fragments are added to the pouring jet according to the invention, the graph assumes the form shown in FIG. 6.

The deformation zone and the cracking zone no longer overlap, and there is an interval of time during which the removal from the mold can be performed without deformation and without cracking. This interval is from 3 minutes to 7 minutes counted from the commencement of pouring, in the case of parts having the above indicated composition and dimensions.

The length of this interval varies with the volume of solid fragments added. 011 the other hand the division of the shrinkage hole varies at the same time. The proportion of solid fragments added will therefore be chosen experimentally in order to obtain simultaneously a suitable interval and good dispersion of the shrinkage hole.

In the case of the compositions indicated hereinbefore, it was found that the optimum proportion was of the order of 1/ 7.

When a molding is cut with a diamond saw, the sawn faces are found to have an appearance similar to that illustrated in FIG. 7. The calibrated fragments are visible at 16. They are seen to be distributed throughout the cross-section of the part. They are perfectly welded to the mass, which must be a result not only of the superficial melting of these fragments, but also of the formation of solid solutions.

There is no large concentrated shrinkage cavity, but a large number of small cavities 17 are observed which are generally self-contained. Small holes 18 due to the gas bubbles are also observed. The molding is also covered with a hard solidification crust, except for the irregularities which may exist on the pouring face due to the removal of the funnel.

FIG. 7 shows by way of example a molding, of which two opposite large faces are convergent, which is appropriate for an arch stone. To manufacture such as molding, two faces of the mold must of course have the required inclination.

When a floor, a wall or an arch for the lining of metallurgical appliances is constructed by assembling such moldings, the joint faces of the parts will all be faces covered by the solidification crust. This makes the lining more suitable to withstand physico-chemical actions.

Obviously the same does not apply when the parts are produced by sawing from a large ingot, because in that case two of the point faces are sawn faces.

A mode of regulating this process will now be described, which will clearly show the combined effect of its special features.

For obvious considerations of homogeneity in the part, the proportion of calibrated solid fragments has an upper limit which is rapidly reached, the suitable proportion being approximately 9 kg. for approximately 60 to 70 kg. of molten oxide poured into the mold, as indicated hereinbefore.

The applicant has found experimentally that a decrease in the proportion of calibrated solid fragments, or of the gas generating substance or substances, tends towards the formation in the solidified ingot, close to the pouring face, of a fairly large shrinkage hole, which may be prejudicial to the mechanical strength of this part of the molding. On the contrary, the applicant has found that an increase in the said proportion of the calibrated solid fragments or of gas generating substances tends to produce, a few seconds after the pouring funnel is removed, a foaming accompanied by a rise of the molten material. The latter then spreads on the pouring face where it solidifies forming a fairly high irregular excrescence which spoils the appearance of the molding and may be a nuisance in its use eg for placing an adjacent part when it is desired to construct linings in several layers.

Generally speaking, this excrescence can be removed by chipping, but this has the disadvantage of requiring a manual operation.

The applicant has found that the quantity of gas generating substance added to the charge to be meltedi.e., the quantity of gas contained in the poured liquidfor a given proportion of calibrated solid fragments can be regulated so as to obtain, after removing the pouring funnel, a slight froth which stops short of forming an excrescence. In this Way the presence of a relatively large shrinkage cavity close to the pouring face is reliably avoided.

It is also possible to prevent the rising of the liquid and the formation of any appreciable excrescence by placing on the pouring scar, immediately after removing the funnel, a cooled or uncooled metallic part with a plane support surface and weighing approximately 1 to kg. per dm. of surfaceJThis buffer part prevents the liquid from emerging and cools it. In this way a molding is obtained without a large shrinkage hole close to the pouring face and not displaying an excrescence of more than a few millimetres. The buffer part may also be removed approximately seconds after having been placed, since solidification is fairly rapid in contact with it.

Two examples are given non-limitatively hereinbelow.

These examples assume that the proportion of calibrated solid fragments introduced into the pouring jet is that given hereinbefore, and that, as gas generating substance, a chromite is used having an ignition loss such as Turkish chromite, in addition to a chromite with no ignition loss such as Transvaal chromite,

They start from the following composition for the magnesia and the chromites which are included in the charge to be melted:

MgO Cr O; SiO; CaO FeO A120 Magnesia 93.8 2. 2 1.8 1. 5 0.7 Chromite with no ignition loss (Transvaal) 11.5 45.0 2.8 0.6 25. 0 15.1 Chromite with 3% ignition loss (Turkish) 16.8 51. 4 3. 9 0.6 14.6 12.7

FIRST EXAMPLE As a buffer piece is not used, the Turkish chromite with 3% ignition loss will be used in a proportion from approximately 15 to 16%.

A suitable composition for the charge will be:

Percent SECOND EXAMPLE If a buffer part is used, the proportion of Turkish chromite with 3% ignition loss may without difficulty be raised to 18% and even up to 20 and which multiplies its bracket of usefulness in a ratio of 3 to 10, giving greater flexibility of regulation and greater certainty of obtaining a regular product.

8 The appropriate composition will be as follows:

P ercent Magnesia Approximately 52. Transvaal chromite Approximately 31.5. Turkish chromite Approximately 16.5.

which will give the following composition to the final product:

I P ercent MgO .z Approximately 55.17. Cr O Approximately 22.66. SiO Approximately 2.67. CaO Approximately 1.22. FeO Approximately 11.06. A1 0 Approximately 7.22.

It is of course obvious that the embodiments described have been given solely by way of example and that they could be modified without thereby departing from the ambit of the invention.

What is claimed is:

1. An improved process of manufacturing a fused cast refractory shaped part by melting a batch of oxide-based refractory material and casting the molten material into a metal mold externally cooled by a circulating liquid wherein said material is left to solidify superficially, the solidification phenomenon being conspicuous by two dwell times of the material in the mold, namely: a first dwell time prior to which distortions of the part would occur upon withdrawal thereof from the mold, and a second dwell time subsequent to which cracks in the part would occur if the same remains in the mold, wherein the improvement comprises the steps of (a) melting a batch containing a substance generating gas at the temperature of the molten material, the gas generating substance being present in the batch in an amount such as to produce a slight frothing on the upper face of the just cast material, selected from the group constituted by the chrome ores having a loss on ignition of more than 1%, calcium sulfate, calcium carbonate, magnesium sulfate, magnesium carbonate, hydrated magnesia and mixtures thereof, (b) casting the same while introducing solid fragments of like oxide-based refractory material into a swirling casting jet throughout the casting step to achieve a substantially homogeneous distribution of said fragments in the case material said solid fragments being added in an amount ranging from a minimum amount allowing the cast part to be rapidly removed from the mold without cracking or subsequent deformation to a maximum amount resulting in a non-homogeneous part, whereby said first dwell time is reduced to a magnitude smaller than said second dwell time leaving therebetween a time interval during which distortions of the part upon withdrawal from the mold no longer occur and cracks in the part do not yet occur, and (c) withdrawing said no longer distortable part from the mold within said time interval whereby cracks thereof are precluded.

2. An improved process as claimed in claim 1, wherein the solid fragments or lumps have substantially the same composition as that of the batch.

3. An improved process of manufacturing a fused cast refractory shaped part by melting a batch comprising magnesia and chromium oxide as major components and casting the molten material into a metal mold externally cooled by a circulating liquid wherein said material is left to solidify superficially, the solidification phenomenon being conspicuous by two dwell times of the material in the mold, namely: a first dwell time prior to which distortions of the part would occur upon withdrawal thereof from the mold, and a second dwell time subsequent to which cracks in the part would occur if the same remains in the mold, wherein the improvement comprises the steps of (a) melting a batch containing from 15% to 25% by weight of Turkish or Iranian chromite as asubstance generating gas at the temperature of the molten material, (b) casting the same While introducing solid fragments having a size ranging from 10 to 20 mm. and a composition substantially similar to that of the batch into a swirling casting jet throughout the casting step to achieve a substantially homogeneous distribution of said fragments in the cast material, the weight ratio of the introduced fragments to the cast material being approximately equal to 1:7, whereby said first dwell time is reduced to a magnitude smaller than said second dwell time leaving therebetween a time interval during which distortions of the part upon withdrawal from the mold no longer occur and cracks in the part do not yet occur, and (c) withdrawing said no longer distortable part from the mold within said time interval whereby cracks thereof are precluded.

References Cited UNITED STATES PATENTS 10 JOHN H. MILLER, Primary Examiner US Cl. X.R. 

